In the field of laser technologies there is a constant need to create ever more powerful lasers while maintaining laser beam quality. The quality of a laser beam is typically measured by how tightly a laser beam can be focused on a target. A diffraction limited beam is a beam that is of the best quality and thus may be tightly focused on a target. One way to increase beam power while maintaining the quality of a laser beam is through Spectral Beam Combining (SBC). Spectral Beam Combining involves combining a plurality of laser beams with differing wavelengths into a single beam using a spectrally dispersive element.
Spectral beam combining may also be used in the field of Wavelength Division Multiplexing and Demultiplexing (WDM). Wavelength Division Multiplexing fiber optic systems transmit multiple wavelength signals through a single fiber over long distances. At the transmitter end, receiver end and add-drop nodes, wavelengths need to be combined (multiplexed) and/or separated (demultiplexed). Wavelength Division Multiplexing and demultiplexing should be done with a large acceptance bandwidth, that is, such that the optical loss through the device does not vary over a large wavelength range.
Present SBC techniques produce diffraction limited beams only for lasers with narrow and precisely fixed wavelengths. For broadband lasers, when performing SBC different spectral components in each beam are dispersed in different directions and the resultant output beam quality is degraded. This is known as spatial chirping. Further, when performing SBC using a laser with a wavelength that is not well controlled or drifts over time, the combined beam will not be diffraction limited. Present SBC designs mitigate this problem by widely separating wavelengths between adjacent elements relative to the laser wavelength drift. As a result present SBC techniques cannot pack wavelengths tightly together resulting in poor utilization of available spectrum, or low spectral density utilization. Therefore, a goal in the field of art is to combine beams with large acceptance bandwidth, or high spectral density, capable of combining large number of elements to produce a powerful diffraction limited output beam.